pns and usg basic

1. BASICS OF PERIPHERAL NERVE
STIMULATOR AND ULTRASOUND
Presented by-Dr.Hrishikesh Bharali,PGT
Moderated by-Dr.Priyam Saikia,
Asst. Proff
Deptt. of Anaesthesiology and Critical Care,
GMCH

2. INTRODUCTION
• “Regional anaesthesia always works, provided you put the
right dose of the right drug in the right place”~Denny Harrop
• Historically, nerve blocks were performed using anatomical
landmarks as a guide as to where to insert the needle and then
eliciting paraesthesia.
• Multiple attempts to elicit paraesthesia which was percieved as
painful, failure rate of nearly 20%, and fear of neurological sequelae
prompted search for safer guidance.
• The advent of Peripheral Nerve Stimulation (PNS) and later
Ultrasound guidance have sought to add an objective end point to
aid nerve location.

3. PERIPHERAL NERVE STIMULATION
• PNS in regional anesthesia is a method of using
a low-intensity and short-duration electrical
stimulus to obtain a defined response (muscle
twitch or sensation) to locate a peripheral nerve
or nerve plexus with an needle and deposit local
anaesthetic around the nerve or in that
compartment to provide a sensory and motor
block for surgery and/or eventually analgesia for
pain management..

4. HISTORY
• 1780 : GALVANI described the effect of Neuromuscular stimulation
• 1912 : PERTHES developed and described Electrical nerve stimulator
• 1955 : PEARSON – concept of Insulated needles for nerve location
• 1962 : GREENBLATT & DENSON – Portable
variable current output nerve stimulator
• 1984 : FORD – Lack of accuracy with
noninsulated needles , Suggested the use of
Constant current nerve stimulator

5. NERVE STIMULATOR

7. COMPONENTS OF PNS
Display
Constant current
generatorControls
Clock reference
Microcontroller
Synchronize
various
functions of
the device
Frequency,du-
ration,current
intensity on
display panel
Display of
adjustalble
variables
Delivers
same
current in
the face of
altering
impedance

8. ELECTROPHYSIOLOGY
• Nerve cells have a resting membrane potential of -90
mV.
• When a neuron is “stimulated” a transient change in
the ion permeability of the membrane (an increase in
the conductance of the sodium channels) occurs. If the
stimulus is strong enough it depolarises the membrane
sufficiently to set off an action potential which then
propagates along the nerve to stimulate the muscle and
causes a contraction.

9. ELECTRICAL STIMULUS
• A certain minimum current intensity is necessary at
a given pulse duration to reach the threshold level
of excitation.
• Rheobase: The lowest threshold current required
to initiate an action potential in the nerve is called
the Rheobase.
• Chronaxie : Chronaxie is the length of time the
current must be applied to the nerve to initiate an
impulse when the current level is twice the
rheobase. Chronaxie values provides an indicator of
the relative excitability of a nerve.

10. Chronaxie of different nerves
NERVE FEATURE CHRONAXIE-ms
C Unmyelinated 0.40
Aδ myelinated 0.17
Aα myelinated 0.05 - 0.10
10

11. • Because of the relatively low capacitance of their myelinated membrane
(uninsulated nodes of Ranvier are the only places along the axon where ions are
exchanged : Saltatory conduction).
• Possible to stimulate a motor nerve but not the sensory nerve by
using a current of smaller chronaxie (shorter time) . This means a
motor response can be seen without producing pain-----however patient
still feels TINGLING.

12. Current INTENSITY
 Current intensity (I) is a measure of stimulus strength and is the flow of
electrical charges used to depolarize the nerve and subsequently produce
a motor response, or ‘‘twitch.
 The delivered current is described by Ohm’s law:
V = I x R (or) I = V / R
where V is voltage( kV) ; I, current (mA) ; R, resistance(kΏ)
• Resistance (R) is primarily independent of the stimulator and is largely a
function of tissue impedance encountered by the needle, poor
connection of the return electrode., Connecting wires.
• Modern nerve stimulators maintain a constant current by raising or
lowering the voltage (V) in response to changes in resistance.

13. FREQUENCY
• Frequency refers to the number of repeating
events per unit time.
• The ideal electric parameters for comfortable
stimulation is 1 to 2 Hz.
• A higher frequency will give more frequent
feedback to the operator, but often causes
greater discomfort to the patient.
Cycles/second

14. NEEDLE TO NERVE DISTANCE
• COULOMB’S LAW: E= K(Q/r2)
where, E is the stimulus intensity
K is a constant
Q is the minimum current from the
needle tip
r is the distance of the stimulus
source from the nerve.
Rearranging the equation, Q ∝ r2
• Hence, at a low current intensity, the nerve will only
be stimulated when the needle is very close to it and
therefore the ability to stimulate the nerve at a very
low current is an indication of proximity to the
nerve.

15. POLARITY “negative to needle,positive to
patient”
• When negative current is applied to the surface
of a nerve, there is resultant depolarisation
leading to generation of an action potential.
• It is better to have the needle as the cathode
because if the needle is positive (the anode) then
the nerve will get hyperpolarised and a larger
current will be needed to depolarise the nerve
and obtain a response.

16. STIMULATING NEEDLES
• Insulated needles are coated with a layer of
non conducting material – Teflon or silicon
• Upon stimulation , the current density
focusses on needle tip
• Hence, a low threshold current is sufficient to
stimulate the target nerve

17. STIMULATION AND INJECTION TECHNIQUE
17
Desired initial:
Current:1-2mA
Pulse duration:0.1 ms
Frequency:1-2Hz
Current gradually reduced & needle advanced
slowly once sought after muscle response obtained
Threshold: 0.2-0.5 mA
at 0.1ms
Aspirate~test dose
of 1-2 ml LA injected
which abolishes
muscle twitch
Failure of the twitch to
disappear, pain on injection of
solution or high injection
pressures suggests intraneural
placement of the needle tip
and warrants small
withdrawal of the needle tip
(0.5 -1mm)
Increase the
current to
initial level
No stimulatory
response-inject
the remaining
drug

18. IDEAL ELECTRICAL CHARACTERISTICS OF A
PNS
Constant current (DC)generator
Monophasic rectangular output pulse i.e. the current flows in one
direction only.
Ability to vary pulse duration (0.1 - 1ms)
Digital display of actual flowing current
Safety features like
• circuit disconnection alert,
• impedence alerts,
• low battery and
• malfunction alert
Leads should be clearly marked to avoid confusion as to which is cathode
and anode
18

19. BASICS OF ULTRASOUND

20. ULTRASOUND FOR ANAESTHESIOLOGISTS
Current and potential future applications of US in anesthesiology are
summarized as follows:
(1) regional anesthesia;
(2) neuraxial and chronic pain procedures;
(3) vascular access;
(4) airway assessment;
(5) lung ultrasound;
(6) ultrasound neuro-monitoring;
(7) gastric ultrasound;
(8) focused transthoracic echo (TTE);
(9) transesophageal echo (TEE) and Doppler

21. ULTRASOUND principle
Sound waves at higher frequencies than can be detected by human
ear (>20,000 Hz).
Medical ultrasound : very high frequency (1-20 MHz).
 US machines utilize the pulse echo principle
US waves are created by a vibrating crystal within a ceramic probe->
transmitted into the patient --> echoes return from various tissue
interfaces --> detected by probe --> processed by computer -->
visualised as an image on screen

22. ULTRASOUND MACHINE PARTS
1. Transducer probe
2. Central Processing Unit(CPU)-Computer that
does all of the calculations
3. Transducer pulse controls- changes the
amplitude,frequency and duration of the pulses
emitted from the probe
4. Display- displays the image from the data
processed by the CPU
5. Keyboard
6. Disk storage devices (hard,floppy,CD)
7. Printer

23. Signal Intensity
• Determined by degree of reflected waves
returning to the transducer
• Larger intensities = Strongly reflected =
Hyperechoic image (Whiter)
• Weaker intensities = Weakly Reflected =
Hypoechoic (Darker)
23

24. et al
Ultrasound Wave Interaction with
Tissues
• Reflection
▫ SPECULAR (large
smooth objects like
a needle) (d)
▫ SCATTERING (most
neural images) (a)
• Refraction (c)
• Transmission (b)

25. Reflection of Ultrasound Waves
• Proportional to the difference in acoustic
impedance between adjacent tissues
• Greater difference = better distinction = better
resolution
• Explains the varying appearances of nervous
tissue on U/S imaging
 Interscalene vs Popliteal

26. Refraction
• Occurs at tissue interfaces (unreflected)
• Refraction can diminish image quality
• Increases with angle of incidence
▫ Optimal angle of incidence is 90°
• Perpendicular probe minimizes effect

29. Attenuation
• Progressive loss of energy with signal
propagation
• Results in progressive decrease in returning
signal
• Major source is conversion of acoustic energy to
heat
• Loss of signal is directly related to depth
• High frequency results in greater attenuation

30. Overcoming Loss of Signal from
Attenuation
• Artificial Enhancement (Adjusting Gain): the
intensity of the reflected sound wave is amplified
• Time Gain Compensation
 Adjusts gain independently at specified
depth intervals
 Most modern U/S machines do this
automatically (autogain)
• Choosing lower frequencies for deeper tissues
(posterior sciatic)

31. RESOLUTION
Resolution refers to the ability to distinguish
one object from another.
 Types:
Spatial-smallest distance that two target can be
separated for the system to distinguish between
them.
Two components- Axial
Lateral
Temporal

32. Axial resolution
• The minimum
separation between
the structures the
ultrasound beam can
distinguish parallel
to its path.
• Determinant :
Size of wavelength-
high frequency
wavelength better .

33. Lateral resolution
• Minimum separation between structures
the ultrasound beam can distinguish in a
plane perpendicular to its path.
• Determinants:
Beam width-smaller the better
Temporal resolution
• Ability of system to accurately
track moving targets over
time.
• Determined by Frame rate

34. Frequency Summary
High frequency
• Improved resolution
• Depth of penetration less
• For superficial structures
• Low frequency
• Poorer resolution
• Depth of penetration
more
• For general
abdominopelvic uses

35. PROBE
(TRANSDUCER)

36. Every probe has an orientation marker that correlates
with another marker displayed on the ultrasound screen

37. ULTRASOUND GEL
Ultrasound gel is a type of conductive
medium that enables a tight bond
(acoustic seal) and removes air
between the skin and the probe or
transducer, letting the waves transmit
directly to the tissues beneath and to
the parts that need to be imaged.

38. MODES of ULTRASOUND
• A-mode (A=amplitude):Oldest
mode.Amplitude of reflected ultrasound is
displayed as one dimensional image.Currently
used only in opthalmology.
• B-mode(B=brightness):a two dimensional
mode and provide a cross sectional image
through the area of interest and is the primary
mode currenly used in regional anaesthesia.
• M-mode (M=motion):one dimensional mode
against time mostly used for cardiac imaging.

39. • D-mode(D=doppler):measures the shift in
the frequency between the incident and the
reflected wave after hitting a moving object.
 Color Doppler
Continuous Wave Doppler
Pulse wave Doppler
Dopler Duplex

40. Frequency Summary
High frequency
• Improved resolution
• Depth of penetration less
• For superficial structures
• Low frequency
• Poorer resolution
• Depth of penetration
more
• For general
abdominopelvic uses

41. All Ultrasound Guided Blocks Involve
Three Steps:
• Choosing One of Two Imaging Views
▫ Short Axis View
▫ Long Axis View
• Scanning the Nerve Track for Image
Optimization
• Choosing a Needle Approach Technique
▫ In Plane
▫ Out of Plane

42. Imaging Plane Options
• Long Axis View or Longitudinal View
▫ Rarely Used
• Short Axis View or Transverse View
▫ Most Commonly Used
▫ Relatively Easy
▫ Better Resolution of Fascial Barriers that Surrond Nerves
▫ Dynamic Assessment of Circumferential Local
Anesthetic Spread
▫ Workable Image Even with Slight Movement of
Transducer Probe

43. Where is The Needle Coming From?
• Out of Plane Technique
Inserting the needle so that it crosses the plane of
imaging near the target.
• In Plane Technique
Inserted within the plane of imaging to visualize the
entire shaft and tip.

44. Out of Plane Approach

45. In Plane Approach

46. Out of plane In plane
• Advantage:
▫ Shorter Needle Insertion
Paths
▫ Less Patient Discomfort
▫ Easier to Perform
• Disadvantage:
Unable to accurately track
needle tip
• Advantage:
▫ Ability to Track the Needle
Tip
▫ Theoretically Safer
• Disadvantage:
▫ More Time Consuming
▫ More difficult to perform
▫ Can be more painful
secondary to longer
insertion paths